Central heating installations of this type are known, in which the hot tap water circuit includes a second duct inside of which flows the heating water, and which extends through the fin assembly in intimate heat contact with the first duct inside of which flows the heating water.
French patent No. 1 352 607 discloses such a heating installation in which the intimate thermal connection between the first duct inside of which flows the heating circuit water and the second duct inside of which flows the tap circuit water is obtained by joining the two ducts in such a manner that they form a double passage tube extending through the fin assembly, this tube having a cross section of appropriate shape, for example circular or elliptic.
By BE-A-671 097 are also known double duct heating installations in which the intimate thermal contact between the ducts, respectively for the heating water circuit and for the tap water circuit, is obtained by incorporating one of the ducts inside the other, these two ducts being then provided in the form of two tubes, possibly concentrical, extending through the fin assembly of the heating apparatus, the outer surface of the inner tube being jointly applied against the inner surface of the outer tube.
Experience shows that in the heating apparatus of the hereabove mentioned type, it is not possible to obtain a perfect thermal exchange between the two ducts since the contact surface between the water conveyed by the heating duct and the tap water duct placed inside the heating duct is too limited. The result is that when there is a non-circulation of water in one of the ducts for an extended period, its temperature can exceed that of the water in movement, and the totality of the heat energy collected by the water which is not circulating is then not transmitted to the water in circulation. The result is that with these known installations it is necessary to have the heating water circulated by starting a heating circuit accelerator in order to obtain an immediate heating of the tap water, thereby compelling the provision of a separate circuit for this circulation when the central heating installation is not in service (as for example during summer).
In order to mitigate these difficulties, it has been envisaged--FR-A-1 484 821-- to form a heat exchanger in which the heating circuit, when extending through a fin assembly, is provided in the form of a plurality of flattened tubes connected to one another by shoulders, and the tap water circuit is provided in the form of a tube bent in the shape of an S or a hairpin, and placed inside the flattened tubes forming the heating circuit duct.
In such a known installation, the rectilinear walls of the tap water circuit tube are plated and generally soldered against the corresponding walls of the heating water circuit flattened tube in which is positioned the tap water circuit. This arrangement has a serious disadvantage: in fact, the physical contact between the two tubes can lead to the boiling of the tap water since, in that case, there is a direct thermal contact between the hot gas flowing through the fin assembly and the two tubes forming the tap water and heating water circuits.